TECHNICAL FIELD
[0001] The present invention relates to a multi-channel optical modulator used as an external
modulator for a laser recording apparatus such as laser printer and laser plotter.
BACKGROUND ART
[0002] A multi-channel optical modulator has a plurality of elastic wave generating sources,
and the number of light beams corresponding to its number can be modulated simultaneously
and independently, which allows to record at a higher speed by scanning at the same
speed as when recording by a single light beam. Further, at the same recording speed,
the recording density is higher than when using a single light beam. Therefore, the
demand for multi-channel optical modulator is increasing along with the mounting requirement
for recording at higher density and higher speed.
[0003] A conventional multi-channel optical modulator is described below.
[0004] As shown in Fig. 4 and Fig. 5, a first electrode 103 is formed on the entire surface
of one side of an acoustic-optical medium 101, and a piezoelectric element 102 is
disposed thereon. Five second electrodes 104 are provided on the piezoelectric element
102, and thereby five transducers are formed.
[0005] A first lead wire 105 is connected nearly to the center of each second electrode
104, and a second lead wire 106 corresponding to each first lead wire 105 is mutually
connected to both ends of the first electrode 103. The first lead wire 105 and second
lead wire 106 are connected to each driving signal source 107.
[0006] In thus constructed multi-channel optical modulator, the operation is described below.
[0007] First, the piezoelectric element 102 is oscillated by an alternating-current signal
supplied from the first lead wire 105 and second lead wire 106, and becomes an elastic
wave generating source. Therefore, the acoustic-optical medium 101 has as many elastic
wave generating sources as the number of transducers, that is, five. The generated
elastic wave propagates vertically on the transducer mounted surface of the acoustic-optical
medium 101, and acts on the light beam passing through the propagation area, thereby
generating a diffracted light. This mode is shown in Fig. 7. Herein, "I" denotes an
incident light, "I1" is a diffracted light, and "I0" represents a non-diffracted transmission
light. Since the diffracted light intensity is proportional to the elastic wave intensity,
that is, the driving signal strength, desired optical recording is realized by varying
the driving signal strength depending on the recording pattern.
[0008] Components of this multi-channel optical modulator are expressed in a circuit diagram
in Fig. 6.
[0009] Herein, the first lead wires 105 and second lead wires 106 are indicated by coil
symbols because they have a very slight inductance. Reference numeral 108 indicates
an output impedance of the driving signal source 107, and the driving signal sources
107 are commonly grounded by connecting among the transducers. The first electrode
103 is commonly shared among the transducers.
[0010] In this structure, when one transducer is driven, the voltage generated by the inductance
of the second lead wire 106 may drive the output impedance 108 and the first electrode
103, or the piezoelectric element 102 of other transducer through the first electrode
103, thereby generating a crosstalk.
DISCLOSURE OF THE INVENTION
[0011] It is hence an object of the invention to present a multi-channel optical modulator
small in crosstalk.
[0012] To achieve the object, the multi-channel optical modulator of the invention comprises
an acoustic-optical medium, a plurality of first electrodes provided on one side of
this acoustic-optical medium, a plurality of piezoelectric elements provided on the
first electrodes, a plurality of second electrodes provided on the piezoelectric elements,
a plurality of first lead wires connected individually to the second electrodes, and
a plurality of second lead wires connected individually to the first electrodes, in
which the first electrodes are independent of individual transducers, and therefore
the voltage generated in the second lead wire of any transducer may not be applied
to the piezoelectric element of any other transducer, so that a generation of crosstalk
may be prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
Fig. 1 is a perspective view of a multi-channel optical modulator in an embodiment
of the invention, Fig. 2 is its side view, and Fig. 3 is its circuit diagram.
Fig. 4 is a perspective view of a conventional multi-channel optical modulator, Fig.
5 is its side view, and Fig. 6 is its circuit diagram.
Fig. 7 is a diagram explaining the operation of a general one-channel optical modulator.
DETAILED DESCRIPTION OF PREFERED EMBPDIMENTS
[0014] An embodiment of the multi-channel optical modulator of the invention is described
in detail below while referring to the accompanying drawings.
(Embodiment 1)
[0015] First, on the entire surface of one side of an acoustic-optical medium 11, Sn is
deposited by vacuum deposition method such as sputtering method or resistance heating
method, and first electrodes 13 are formed.
[0016] The acoustic-optical medium 11 is made of tellurium dioxide, lead molybdate, or a
similar material allowing to pass light at the wavelength to be modulated, and having
a large figure of merit in the acousto-optic effect. The acoustic-optical medium 11
has parallel upper and lower surfaces, and the first electrodes 13 are formed on the
side vertical to these upper and lower surfaces. The side opposite to the side on
which the first electrodes 13 formed is not parallel to the side on which the first
electrodes 13 formed.
[0017] On the first electrodes 13, a piezoelectric element 12 having a narrower width than
the width of the first electrodes 13 is formed.
[0018] The piezoelectric element 12 is made of lithium niobate which is a piezoelectric
crystal, and it is compression-bonded onto the first electrodes 13 in a vacuum, and
polished to a thickness for resonating at a driving frequency.
[0019] At this time, the piezoelectric element 12 is formed so that its width direction
end may not coincide with the width direction end of the first electrodes 13, that
is, it is deviated in position so that the second lead wires 16 may be formed on the
first electrodes 13 in a later process.
[0020] On the piezoelectric element 12, further, five second electrodes 14 are formed at
specific intervals. The second electrodes 14 have a three-layer structure, formed
of Ni-Cr layer, Cu layer, and Au layer sequentially from the piezoelectric element
12 side.
[0021] Afterwards, four split grooves 18 are formed in the first electrodes 13 and acoustic-optical
medium 11 by using a dicer between the adjacent second electrodes 14, and five transducers
having independent piezoelectric elements 12 and first electrodes 13 are fabricated.
[0022] The first lead wire 15 is connected electrically by soldering to the second electrode
14 of each transducer. At this time, the first lead wire 15 is alternately connected
to the different end from the adjacent second electrode 14. Further, the second lead
wire 16 is soldered to the first electrode 13 at the soldering side of the first lead
wire 15 of each transducer. At this time, for the ease of soldering of the second
lead wires 16, a plating layer 19 of two layers made of lower layer of Cu and upper
layer of Au is formed at the soldering position of the second lead wire 16 of the
first electrode 13.
[0023] Consequently, in each transducer, the first lead wire 15 and second lead wire 16
are connected to a driving signal source 17.
[0024] Thus, the multi-channel optical modulator having five transducers of the same shape
as shown in Fig. 1 and Fig. 2 is formed.
[0025] Fig. 3 is a circuit diagram of this multi-channel optical modulator, in which the
first lead wires 15 and second lead wires 16 are indicated by coil symbol because
they have, if very slight, an inductance. Reference numeral 20 is an output impedance
of the driving signal source 17, and the grounding of the driving signal source 17
connected to each transducer is common.
[0026] Each transducer of this multi-channel optical modulator individually has an independent
first electrode 13, and the signal of a certain transducer will not be applied to
any other transducer through the output impedance 20 or first electrode 13.
[0027] Besides, since the distance between the first lead wires 15 of adjacent transducers
is long, the induction of the inductance is small, so that the crosstalk may be decreased.
The characteristic points of the invention are explained below.
(1) The transducer forming surface of the acoustic-optical medium 11 and its opposite
side are preferred to be mirror-finished surfaces in order to obtain a stable modulation
operation.
(2) The first electrodes 13 are preferred to be formed by using Sn or In in order
to obtain matching of acoustic impedance between the piezoelectric element 12 and
acoustic-optical medium 11.
(3) The split groove 18 is formed so that its section may be in a U-form. It is because,
if there is a sharp corner in the split groove 18, a stress is formed in the area
to cause a crack in the acoustic-optical medium 11. Therefore, the surface forming
the split groove 18 of the acoustic-optical medium 11 is formed in a curved surface.
(4) Usually, the light beam to be modulated enters in a range of 1 to 3 mm from the
transducer. The elastic wave generated from the transducer is propagated radially,
and the angle of radiation of the elastic wave is wider as the interval of the transducers
becomes narrower.
Accordingly, when the transducer interval is narrow, the incident light is diffracted
by the elastic wave generated by the adjacent transducer, thereby causing an crosstalk.
Therefore, by setting the depth of the split groove 18 deeper than the light beam
incident position, if the transducer interval is narrow, it is possible to prevent
crosstalk generated by the elastic wave transmitted from the adjacent transducers.
Incidentally, if free from effects of diffraction by the elastic wave from the adjacent
transducer, the depth is not particularly defined as far as each transducer may exist
independently electrically.
(5) The transducer generates heat by electro-mechanical conversion loss proportional
to the electric input. Hence, deviation of temperature distribution depending on the
holding structure and a shape of the acoustic-optical medium 11 may be generated in
the acoustic-optical medium 11 to cause distortion.
On the other hand, the transducer is lowered in the modulation efficiency for a specific
electric input if increased in the width in the vertical direction.
Therefore, in the case of the multi-channel optical modulator having a plurality of
transducers, the transducer in the portion lower in the temperature of the acoustic-optical
medium 11 is, as compared with the transducer in the portion higher in temperature,
set larger in the width in the vertical direction, larger in the electric input for
obtaining an equivalent efficiency, and more in the heat generation due to electro-mechanical
conversion loss. Therefore distortion of the acoustic-optical medium 11 can be prevented
without deviation of temperature distribution in the acoustic-optical medium 11 without
varying the modulation efficiency of each transducer.
(6) In order to release heat in the acoustic-optical medium 11, it is preferred to
form cooling plates on the upper and lower surfaces of the acoustic-optical medium
11. The shape of the cooling plate is not particularly specified as far as the surface
contacting with the acoustic-optical medium 11 is flat.
(7) In this embodiment, by forming the split groove 18, a plurality of transducers
having independent electrodes 13 are formed, but instead of forming the split groove
18, the first electrodes 13 and piezoelectric elements 12 may be preliminarily formed
independently.
(8) The interval of the second electrodes 14 of the adjacent transducers should be
as equal as possible. This is because if the interval of the second electrodes 14
is different, the crosstalk varies depending on the transducers.
(9) In the foregoing embodiment, five transducers are provided, but the number of
transducers may be determined freely depending on the number of desired channels.
INDUSTRIAL APPLICABILITY
[0028] According to the invention, by forming the first electrodes of the transducers independently
electrically, an excellent multi-channel optical modulator small in crosstalk is realized.
Reference numerals in the drawings
[0029]
- 11
- Acoustic-optical medium
- 12
- Piezoelectric element
- 13
- First electrode
- 14
- Second electrode
- 15
- First lead wire
- 16
- Second lead wire
- 17
- Driving signal source
- 18
- Split groove
- 19
- Plating layer
- 20
- Output impedance
- 101
- Acoustic-optical medium
- 102
- Piezoelectric element
- 103
- First electrode
- 104
- Second electrode
- 105
- First lead wire
- 106
- Second lead wire
- 107
- Driving signal source
- 108
- Output impedance
1. A multi-channel optical modulator comprising:
an acoustic-optical medium;
a plurality of first electrodes provided on one side of the acoustic-optical medium;
a plurality of piezoelectric elements provided on the first electrodes;
a plurality of second electrodes provided on the piezoelectric elements;
a plurality of first lead wires connected individually to the second electrodes; and
a plurality of second lead wires connected individually to the first electrodes.
2. The multi-channel optical modulator of claim 1, wherein a split groove is provided
in the acoustic-optical element between adjacent electrodes of the first electrodes.
3. The multi-channel optical modulator of claim 2, wherein the section of the split groove
is a curved shape.
4. The multi-channel optical modulator of claim 2, wherein the depth of the split groove
is deeper than a light beam incident position.
5. The multi-channel optical modulator of claim 1, wherein
the first lead wires are connected to one end of the second electrodes,
the adjoining first lead wires are connected to other end of the second electrodes,
and
the second lead wires making pairs with the first lead wires are connected to the
first electrodes close to the first lead wires.